1. Field of the Invention
The present invention relates to a digital television receiver which receives compressed video signals and decodes them to display on a monitor, and more particularly to an inverse discrete cosine transforming (IDCT) system which performs an inverse discrete cosine transformation with respect to DCT coefficients inversely quantized.
2. Discussion of Related Art
In recent years, digital television (TV) broadcasting is making rapid progress, and the techniques of compressing and transmitting video data become very important. The moving pictures expert group (MPEG) standards are international ones for compression coding of digital picture, and applied to a digital versatile disk (DVD) and a digital TV.
In the presently available digital TV the MPEG compression and restoration of high definition become the actual standards, which means that the conventional analog TVs come to be replaced with digital TVs gradually. However, since HDTVs are considerably expensive in the early stage, there may be the demand for televisions of standard definition (SD) in the transition from the analog TVs of NTSC to perfect HDTVs for a long period of time. The SDTVs do not display HD signals on an HD monitor but performs a down-conversion with respect to HD signals and display them on SD monitors, i.e. NTSC TVs or PC monitors. This SDTV can also receive SD signals. The SDTV can convert 1920 pixels.times.1080 lines, 60 Hz interlaced scan HD signals into 720 pixels.times.480 lines,60 Hz interlaced scan SD signals for broadcasting, and may use 720 pixels.times.480 lines, 60 Hz interlaced scan SD signals without conversion.
Digital TV receivers for SDTV are divided into HD and SD levels according to the number of pixels considering factors influencing the sharpness/picture quality. A transmitting part absolutely requires the removal of a temporal redundancy as well as the removal of a redundancy in the two-dimensional space that the video data has in order to efficiently compress the video bit stream that varies with the time. For example, the MPEG employs the motion compensation technique to recude the temporal redundancy and DCT to reduce the redundancy in the two-dimensional space. The picture is divided by blocks by a method of removing data correlation through two-dimensional pivotting, and each block is pivotted by DCT algorithm. The pivotted data tend to one direction (e.g. low-pass direction), and the data are quantized and transmitted.
A decoder of the digital TV for SDTV, is shown in FIG. 1. One pixel is expressed as 8 bits, and one macro block has 16.times.16 pixel data. This decoder receives the bit stream including the motion vector information.
A variable length decoder (VLD) 101 performs a variable length decoding with respect to the applied bit stream and divides it into motion vector, quantization value, DCT coefficients. An inverse quantizer 102 inversely quantizes DCT coefficients produced from VLD 101 and applies it to a demultiplexer 103. Demultiplexer 103 produces the 8.times.8 DCT coefficients inversely quantized in response to a selection signal to an 8.times.8 IDCT 104, and removes a high frequency area of a horizontal part of inversely quantized 8.times.8 DCT coefficients and produces 8.times.4 DCT coefficients to 8.times.4 IDCT 105. 8.times.8 IDCT 104 performs an IDCT in the unit of 8.times.8 with respect to the inversely quantized 8.times.8 DCT coefficients, and 8.times.4 IDCT 105 performs an IDCT in the unit of 8.times.4 with respect to the inversely quantized 8.times.4 DCT coefficients and produces them to an adder 107 through a multiplexer 106.
Adder 107 adds motion compensated data to the IDCT data and restores to a perfect picture to store the restored picture in a frame memory 109. The restored original picture signal is video out for display and simultaneously feeds back to a motion compensator 108 for motion compensation. Motion compensator 108 compensates the motion of the current frame by using motion vectors produced from VLD 101 and pixel values of frame memory 109, and then outputs it to adder 107.
Demultiplexer 103 produces inversely quantized 8.times.8 DCT coefficients to 8.times.8 IDCT 104 if the applied signal is SD one. If the applied signal HD one, after removing a high frequency area of the horizontal part of the inversely quantized 8.times.8 DCT coefficients, demultiplexer 103 produces 8.times.4 DCT coefficient to 8.times.4 IDCT 105.
The current MPEG standards propose two-dimensional IDCT of 8.times.8 block. Thus, 8.times.8 IDCT 104 and 8.times.4 IDCT 105 perform two-dimensional IDCT. 8.times.8 2-D IDCT equation is expressed as equation 1.
##EQU1##
wherein u,v,x,y=0,1,2, . . . , 7 ##EQU2##
wherein x and y are coordinates in PEL domain and u and v are a coordinates in transform domain.
8.times.8 2-D IDCT 104 which processes DCT coefficients of standard definition includes one-dimensional 8.times.1 IDCT operation part 202, a transposition part 203, and a one-dimensional 8.times.1 IDCT 204, as shown in FIG. 2a.
A one-dimensional 8.times.1 IDCT operation part 202 of 8.times.8 2-D IDCT 104 of FIG. 2a performs a one-dimensional IDCT with respect to DCT coefficients of 8.times.8 block applied through demultiplexer 103, and produces it to transposition part 203 as shown in FIG. 2b. Transposition part 203 performs a column-row, i.e. a horizontal-vertical transposition. One-dimensional 8.times.1 IDCT 204 performs one-dimensional 8.times.1 IDCT of an output of transposition part 203 in the vertical direction, as shown in FIG. 2c, thus completing 8.times.8 two-dimensional IDCT and obtaining an image block 205.
One-dimensional 8.times.1 IDCT operation part 204 in FIG. 2a includes an 8.times.1 IDCT operation part 206 and a rounding part 207, as shown in FIG. 2d, and the 1-D rounding part is different from that of 2-D according to the MPEG standards.
Since the digital TV receiver for SDTV receives and decodes SD data, there is no trouble in performance of IDCT with respect to SD data by 8.times.8 IDCT 104 , but performing IDCT of HD data is problematic. As described above, the number of HD pixels is six times larger than that of SD pixels. If signals applied to demultiplexer 103 are HD signals, horizontal decimation is performed with respect to 8.times.8 DCT coefficients, thus applying 8.times.4 DCT coefficients to 8.times.4 IDCT 105. When 8.times.4 IDCT 105 performs a two-dimensional IDCT, 8.times.4 2-D IDCT equation is as follows: ##EQU3##
wherein x, u=0, 1, 2, 3, and 4 PA1 y, v=0,1,2, . . . , 7
8.times.42-D IDCT 105, as shown in FIG. 3a, includes a one-dimensional 8.times.1 IDCT operation part 301, a transposition part 302, and one-dimensional 4.times.1 IDCT operation part 303. Similarly, after one-dimensional 8.times.1 IDCT operation part 301 performs one-dimensional IDCT with respect to the applied HD 8.times.4 DCT coefficients, transposition part 302 performs column-row transposition, and one-dimensional 4.times.1 IDCT operation part 303 performs one-dimensional 4.times.1 IDCT, thereby completing 8.times.4 two-dimensional IDCT.
FIG. 3b shows another embodiment of processing HD signals, and HD signals are converted into SD ones through filtering/decimation for downsampling. Signals of pictures affecting the picture quality, are downsampled to 8.times.4, and signals of a picture not affecting the picture quality are downsampled to 4.times.4. Methods of IDCT/downsampling to M.times.N(8.times.4) or N.times.N(4.times.4) are various and FIG. 3b shows one of them.
If the inversely quantized DCT coefficients are picture signals affecting the picture quality, a first IDCT and down-sampling part 305 performs an IDCT of inversely quantized DCT coefficients, and performs a 1/2 downsampling in a horizontal direction to reduce to 8.times.4. The downsampled 8.times.4 DCT coefficients are applied to an adder 306 and added to motion-compensated data to be then applied to a third downsampling part 307 and stored in a frame memory 308. A motion compensator 309 reads a reference frame out of frame memory 308 by using a motion vector MV produced from VLD 101, and performs a motion compensation of the current frame to output it to adder 306.
Third downsampling part 307 performs low-pass filtering and vertical downsampling of 8.times.4 data produced from adder 306, and outputs it to multiplexer 310. A bit reduction encoding part and a bit decoding part may be placed prior to and behind frame memory 308 for reducing the size of frame memory 308. The bit reduction encoding part reduces the bit volume of the applied data by using correlation of the adjacent pixels, and stores the data in frame memory 308. The bit decoding part decodes the data produced from frame memory 308, and restores images for motion compensator 309.
If the inversely quantized DCT coefficients are signals not affecting the picture quality, a second IDCT and downsampling part 311 performs an IDCT of the DCT coefficients, and performs a 1/2 downsampling in a horizontal/vertical direction, thus reducing the 8.times.8 size to 4.times.4.
The downsampled 4.times.4 DCT coefficients are applied to an adder 312, and added to the motion compensated data to be transferred to multiplexer 310 and stored in a frame memory 313. A motion compensator 314 reads a reference frame out of the frame memory 313 by using a motion vector MV produced from VLD 101, and performs a motion compensation, thus transferring it to adder 312. A bit reduction encoding part and a bit decoding part may be placed prior to or behind frame memory 313 for reducing the size of frame memory 313. Decoding with respect to the picture not affecting the picture quality is carried out by repeating the above process.
Multiplexer 310 rearranges output of third down sampling part 307 and adder 312 and produces downsampled video signals of a desired size (video out). This desired size may refer to a picture of 720 pels.times.480 lines, and, in this case, third downsampling part 307 converts HDTV signals of 1080 lines into SDTV signals of 480 lines. When an SDTV receiver using MPEG2 receives and processes HD video signals, downsampling can be performed to make a picture affecting the picture quality of reproduced screen contain much information compared to a picture not affecting the picture quality, thus providing high picture quality.
FIG. 4 is a view for describing video source decoding by progressive scanning, and shows a 1-D IDCT/downsampling.
A horizontal 1-D IDCT/filtering and decimation 402 is performed with respect to image data of one block unit 401 to obtain IDCT and an 8.times.4 image block 403 downsampled, and if the 1-D IDCT and downsampled data are signals of a picture affecting the picture quality, a vertical 1-D IDCT 404 is performed only without downsampling, thereby obtaining an image block 405 of 8.times.4.
If the horizontal 1-D IDCT and downsampled data are signals of a picture not affecting the picture quality, the vertical 1-D IDCT/filtering and decimation 406 are performed to obtain an image block 407 of 4.times.4.
FIG. 5 is a view for describing the video source decoding by interlaced scanning having top and bottom fields, and shows the 1-D IDCT/downsampling (pixels of top and bottom fields are distinguished by black and white dots. That is, a horizontal 1-D IDCT/filtering and decimation 502 is performed with respect to image data of one block unit 501 having top and bottom fields, thus obtaining an 8.times.4 image block 503 IDCT and downsampled top and bottom fields. If the 1-D IDCT and downsampled data are signals with big importance, only vertical 1-D IDCT 504 is carried out without downsampling, thereby obtaining an image block 505 of top and bottom fields of 8.times.4. If the 1-D IDCT and downsampled data are signals of a picture not affecting the picture quality, vertical 1-D IDCT/filtering and decimation 506 is performed to obtain a 4.times.4 image block 507 with top and bottom fields.
FIG. 6a shows another preferred embodiment of 1-D IDCT for processing SD DCT coefficients. A 1-D IDCT 602 performs an JIDCT with respect to applied 8 DCT coefficients, and produces 8 pixels. 1-D IDCT 602 performs an IDCT with respect to the coefficient information for IDCT from a read only memory (ROM) 601. FIG. 6b is still another preferred embodiment of 1-D IDCT for processing HD DCT coefficients. A 1-D IDCT/filtering and decimation part 604 performs IDCT and downsampling with respect to applied 8 DCT coefficients, and produces four pixels. Part 604 receives the coefficient information for IDCT from a ROM 603.
In FIGS. 6a and 6b ROMs 601 and 602 each storing the coefficient information for IDCT provide IDCT coefficient values according to each mode. The 2-D IDCT and downsampling is depicted in FIGS. 2 to 5.
As mentioned above, the standard definition (SD) TV receiver requires two kinds of IDCTs in order to receive SD and HD video signals and display them. That is, as shown in FIGS. 1, 6a and 6b, IDCT for decoding HD video signals and IDCT for decoding SD video signals are individually required, and extra read only memories(ROMs) holding the IDCT coefficient information corresponding to each of two decoding modes must be provided, which increases the manufacturing cost.